Some 13 billion years ago in a distant cluster
of stars, a planet formed. Remarkably it's still there, according
to data from the Hubble Space Telescope.

by Ron Koczor

Long before our Sun
and Earth ever existed, a Jupiter-sized planet formed around a sun-like
star. Now, almost 13 billion years later, NASA's Hubble Space Telescope
has precisely measured the mass of this farthest and oldest
known planet.

The ancient planet
has had a remarkable history, because it has wound up in an unlikely,
rough neighbourhood. It orbits a peculiar pair of burned-out stars
in the crowded core of a globular star cluster.

The new Hubble findings
close a decade of speculation and debate as to the true nature of
this ancient world, which takes a century to complete each orbit.
The planet is 2.5 times the mass of Jupiter. Its very existence
provides tantalizing evidence the first planets were formed rapidly,
within a billion years of the Big Bang, leading astronomers to conclude
planets may be very abundant in the universe.

The planet lies near
the core of the ancient globular star cluster M4, located 5,600
light-years away in the northern-summer constellation Scorpius.
Globular clusters are deficient in heavier elements, because they
formed so early in the universe that heavier elements had not been
cooked up in abundance in the nuclear furnaces of stars. Some astronomers
have therefore argued that globular clusters cannot contain planets,
because planets are often made of such elements. This conclusion
was seemingly bolstered in 1999 when Hubble failed to find close-orbiting
"hot Jupiter"-type planets around the stars of the globular cluster
47 Tucanae. Now, it seems astronomers were just looking in the wrong
place, and gas-giant worlds, orbiting at greater distances from
their stars, could be common in globular clusters.

"Our Hubble measurement
offers tantalizing evidence that planet formation processes are
quite robust and efficient at making use of a small amount of heavier
elements. This implies that planet formation happened very early
in the universe," said Steinn Sigurdsson of Pennsylvania State University.

An artist's
concept of a planet orbiting two stars - a neutron star
and a white dwarf - in the globular cluster M4. The skies
of the densely-packed cluster are remarkably starry.

"This is tremendously
encouraging that planets are probably abundant in globular star
clusters," agrees Harvey Richer of the University of British Columbia
(UBC) in Vancouver. He bases this conclusion on the fact a planet
was uncovered in such an unlikely place: orbiting two captured stars,
a helium white dwarf and a rapidly spinning neutron star, near the
crowded core of a globular cluster. In such a place, fragile planetary
systems tend to be ripped apart due to gravitational interactions
with neighbouring stars.

The story of this planet's
discovery began in 1988, when the pulsar, called PSR B1620-26, was
discovered in M4. It is a neutron star spinning just under 100 times
per second and emitting regular radio pulses like a lighthouse beam.
The white dwarf was quickly found through its effect on the clock-like
pulsar, as the two stars orbited each other twice per year. Sometime
later, astronomers noticed further irregularities in the pulsar
that implied a third object was orbiting the others. This new object
was suspected to be a planet, but it also could have been a brown
dwarf or a low-mass star. Debate over its true identity continued
throughout the 1990s.

Sigurdsson, Richer,
and their co-investigators settled the debate by at last measuring
the planet's actual mass through some ingenious detective work.
They had exquisite Hubble data from the mid-1990s taken to study
white dwarfs in M4. Sifting through these observations, they were
able to detect the white dwarf orbiting the pulsar and measure its
colour and temperature. Using evolutionary models computed by Brad
Hansen of the University of California, Los Angeles, the astronomers
estimated the white dwarf's mass.

This in turn was compared
to the amount of wobble in the pulsar's signal, allowing the team
to calculate the tilt of the white dwarf's orbit as seen from Earth.
When combined with the radio studies of the wobbling pulsar, this
critical piece of evidence told them the tilt of the planet's orbit,
too, and so the precise mass could at last be known. With a mass
of only 2.5 Jupiter's, the object is too small to be a star or brown
dwarf and must instead be a planet. The planet is likely a gas giant
without a solid surface like the Earth.

A 13-billion year old
planet orbiting a pair of long-dead stars in a crowded globular
cluster: even for the Hubble Space Telescope, that's amazing!